Adam and Eve had a choice, just as the angels had a choice. That is what mortal sin is really all about - choosing to separate oneself from God. We were all born with the original sin of Adam and Eve, but we also have a choice whether or not to sin ourselves. After baptism, we are cleansed and then begins our responsibility to avoid the temptation to commit a grave and evil act. Everyone seems to commit venial sins, simply because we are imperfect, but God allows for that, understands that and forgives that. He also forgives mortal sins, and that is what confession is really for. People confess their venial sins as well (nuns have regular confession, as do most religious and priests) but to me, avoiding mortal sin seems to be a case of simply not indulging in acts that I know to be serious and evil and that would separate me from God. If there is temptation, the first step is to remove oneself from that occasion of temptation. That takes effort perhaps, but once the temptation is removed, then it gets much easier not to act upon it. Like most things in life, avoiding mortal sin requires one to accept responsibility for their own actions. Blaming the devil is laziness because it excuses one from their own actions. ‘The devil made me do it!.’ That is why some people have trouble stopping a sin - they act as if it were beyond their own efforts. But God doesn’t make it hard for people to be good, He makes it hard for them to sin (at least mortal sin). He gives three conditions to be met, and at any time a persona can check themselves with these three conditions and ask themselves, ‘Is it grave? Am I aware that it is grave? Do I consent to this grave and evil deed?’
I have compassion for those who find avoiding sin hard, but I do believe it is like alcoholics anonymous a little (just an example) in that the person must take a step away from the temptation every single day and continue doing so one day at a time. But the will must be aligned with the consequences, asking oneself, ‘What will be the ultimate outcome if I follow this path?’ Only a child is excused from knowledge of consequences, and they learn this over time. An adult should know that every action has consequences, and they should align their will with the consequences that they know will be right and good. Then it is so much easier to avoid actions that will lead to bad consequences.
One thing that really helps avoid sin is a sincere and deep love for God, combined with a personal relationship with Jesus - whatever that means to the individual. Then sinning becomes real, a personal affront to a friend rather than an abstract evil against a forgiving God. If you were with Jesus in person, just hanging out, you might feel less likely to go sin, but if you did feel the urge, you could talk about it with Him - the chances are you would be less likely to leave Him then to go do something you know is wrong and would offend Him.
The good news though is that even after a person has committed a mortal sin, God will not reject them if they turn to Him for forgiveness.
